A considerable effort has been made to develop processes for rapidly and readily depositing amorphous or substantially amorphous alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment. This work was originally concentrated in the field of amorphous silicon alloys. Thin films may be deposited by plasma enhanced chemical vapor deposition. In plasma enhanced chemical vapor deposition, also referred to as glow discharge deposition, a hydrogen containing feed stock gas, such as a hydride or polyhydride, is decomposed in the plasma by electron collisions to form unstable precursor species, i.e., molecular fragments, principally hydrogen containing free radicals. These unstable precursor species, as ions, or free radicals, diffuse to the substrate surface and deposit thereupon as a film through surface reactions involving partial hydrogen elimination.
In the prior art processes, the substrate temperature is the greatest single determinant of the hydrogen content of the deposited film. This is because the hydrogen elimination reaction is strongly dependent on the substrate temperature, and is more efficient at high substrate temperatures. In turn, the deposited film's optical, electrical, and mechanical properties are known to be strongly dependent on the hydrogen content and bonding type. For example, amorphous silicon deposited by conventional plasma assisted chemical vapor deposition from silane feedstock onto a cold substrate has a high hydrogen content, many (Si--H.sub.2).sub.n bonds, poor abrasion resistance, poor corrosion resistance, and poor semiconductor properties.
Fluorinated feedstock gases have also been used with the glow discharge process to deposit thin films on substrates. Fluorine is a highly reactive element which forms species in the plasma principally fluorine free radicals, which are capable of etching the substrate and the deposited film and can thus prevent the film from forming unless controlled. When fluorinated feedstock gases are used with hydrides or polyhydrides, hydrogen acts as a carrier to combine with excess fluorine to form, for example, HF and thereafter remove some of the fluorine from the plasma. Provided the proportions are properly chosen, this fluorine removal process causes a reduction in the etching process otherwise produced by the fluorine, which balances the deposition kinetics in favor of film growth to allow the deposition to take place. In the process, however, both hydrogen and fluorine are inevitably incorporated in the film. This may be desirable for attaining certain properties in the film, such as those related to semiconductor action, but the heretofore unavoidable incorporation of hydrogen and fluorine in such films, particularly at low substrate temperatures, results the production of films which exhibit other properties totally unacceptable for the intended purpose.
Because of the uncontrolled incorporation of hydrogen and in some cases fluorine in films formed using the glow discharge process, it has not been possible to use this process to make certain alloy films which can be deposited using other processes, such as sputtering or evaporation. Because such other processes, e.g., sputtering, are not economical for large scale manufacturing purposes, due to fundamental limitations on deposition rate, many known films, while having highly desirable properties such as hardness and transparency, have not enjoyed widespread commercial use.
While some hydrogen free films have been deposited by some workers in the field using glow discharge techniques, because of an incomplete understanding of the separate roles played by plasma species, e.g. gassifier, scavenger, etc. exotic plasma conditioning for instance, electron cyclotron resource excited microwave plasmas, or high substrate temperatures were required to achieve the desired film deposition. For example, U.S. Pat. No. 4,481,229 to Suzuki et al discloses the formation of a hydrogen free Si-N film using a microwave glow discharge process occurring in magnetic field of intensity B such as to result in near electron cyclotron resonance conditions at the frequency of the applied microwave radiation. Practical deposition systems utilizing this art therefore will require the creation and control of a uniform large magnetic field throughout the plasma volume. Additionally such a system will require the use of microwave tuning and applicator technology capable of coupling microwave energy into a plasma impedance which is changing rapidly near the resource condition. The additional technological complications brought about by these requirements have significant economic consequences for manufacturing machines employing this art.
In other examples, including (but not limited to) M. Janai, et al "Chemical Vapor Deposition of Amorphous Silicon Prepared from SiF.sub.2 Gas, " J. Appl. Phys. 52 (5), pp 3622-3623; M. Janai, et al "Properties of Fluorinated Glow-Discharge Amorphous Silicon," Phys. Rev. B, 31 (8), pp 5311-5321; M. Janai, et al "Optical Absorption And Photoluminescense of Glow Discharge Amorphous Si:F Films,: Phys Rev. B, 31 (10) pp 6609-6615; K. Kuppers, et al "Deposition of Fluorine Deoped Silicon Layers From A SiCl.sub.4 /SiFy/O.sub.2 Gas Mixture By The Plasma CVD Method," J. Electrochem. Soc: Solid State Science and Technology, Aug. 1978 pp 1298-1302; D. R. Secrist, et al "Glow Discharge Syntheon of A Fluorosiloxane Polymer", Polymer Letters, 4, pp 537-540. S. Fujita, et al "Electrical Properties of Silicon Nitride Films Plasma Deposited From SiF.sub.4, N.sub.2, and H.sub.2 Source Gases," J. Appl. Phys., 57(2), pp 426-431; Fujita, et al "Plasma Enhanced Chemical Vapor Deposition of Fluorinated Silicon Nitride," Jap. J. Appl. Phys. 23(3), pp L144-146, and Fujita, et al "Plasma Deposited Silicon Nitride," Jap. J. Appl. Phys., L268-L28O., disclose the use of halogenated feedstock gases with or in some cases, without hydrogen to deposit nominally hydrogen free films by plasma glow discharge decomposition. In all cases these researchers utilize elevated substrate temperatures to control hydrogen and halogen content.
Many techniques of exciting conventional glow discharge plasmas have been investigated. These have included direct current (DC) and alternating current (AC) techniques. Various AC frequencies have been utilized, such as audio, radio frequency (RF), and a microwave frequency of 2.56 GHz. The present inventors have shown, in U.S. application Ser. No. 605,575, now U.S. Pat. No. 4,504,518 that the optimum deposition power and pressure are defined by the minimum of the Paschen curve. The Paschen curve defined the voltage (V) needed to sustain the glow discharge plasma at each pressure (P) in a range of pressures, between electrodes separated by a distance (D). In a typically configured, conventional RF glow discharge system, the minimum in the Paschen curve occurs at a few hundred Torr.
It has previously been discovered that increasing the applied RF power increases the gas utilization efficiency and the deposition rate. However, simply increasing the RF power to achieve deposition rates approximately greater than 10 Angstroms/sec. leads to the production of hydrogen containing alloy films of decreasing hardness, abrasion resistance, and electronic quality, and can result in films which include polymeric material and/or the production of powder. The increased deposition rate with increased RF power is a result of an increase in the concentration of excited species resulting principally from collisions between electrons and feedstock molecules. However, the collision rate between excited species and more importantly between excited species and feedstock molecules is also increased. This results in the formation of gas phase polymer chains. These chains are either incorporated in the growing alloy film degrading its physical and/or electronic quality, or condensed in the gas phase to produce powder particles. To reduce the number of undesirable collisions one can reduce the operating pressure, but this moves the deposition process off the minimum of the Paschen curve and substantially higher RF power is required to achieve the same degree of plasma excitation. The physical reason for this phenomenon is that, as pressure is reduced, many electrons that would have been able to collisionally excite feedstock molecules at higher gas pressures now impinge on the substrate or system walls without suffering collisions. The much higher applied RF power required to sustain the plasma at these reduced pressures has the undesirable consequence of raising the substrate self-bias with respect to the plasma, thus producing greatly increased ion bombardment of the deposited film.
Attempts have also been made to increase the gas utilization efficiency in RF glow discharge plasmas by high power deposition of a dilute mixture of silane (SiH.sub.4) in an inert carrier gas such as argon. However, this is known to result in undesirable film growth conditions giving rise to columnar morphology as reported by Knights, Journal of Non-Crystalline Solids, Vol. 35 and 36, p. 159 (1980).
The one group which has reported glow discharge amorphous silicon-hydrogen semiconductor alloy deposition utilizing microwave energy at 2.54 GHz treated the microwave energy as just another source of plasma excitation by performing the deposition in a plasma operating at pressures typical of conventional RF or DC glow discharge processes. C. Mailhoit et al. in the Journal of Non-Crystalline Solids, Vol. 35 and 36, p. 207-212 (1980) describe films deposited at 0.17 Torr to 0.30 Torr at deposition rates of between 23 and 34 Angstroms/Sec. They report that their films, which are of poor electrical quality, show clear indication of non-homogeneous structure. Thus, Mailhoit et al failed to find that for a given deposition system the minimum in the Paschen curve shifts to lower pressure values with increasing frequency. Therefore, the use of high frequency microwave energy in a glow discharge deposition system allows one to operate at much lower pressure without production of powder or inclusion of polymeric species in the amorphous semiconductor film. The shift in the minimum of the Paschen curve is believed to occur because, for a given gas pressure at the higher excitation frequency, the rapid reversals of the applied electric field allow the electrons in the plasma to collide with more feedstock molecules in the plasma excitation region before they encounter the walls of the system. Thus, operating at microwave frequencies provides both a substantially increased deposition rate, e.g., 100 Angstroms/sec. or above, and a feedstock conversion efficiency approaching 100%, while still allowing the production of high quality films. This contrasts both with the conventional RF (e.g., 13.56 MHz, 0.2 to 0.5 Torr) glow discharge deposition process which produces high quality films at deposition rates of approximately 10 Angstroms/sec. and feedstock utilization of approximately 10% and with the Mailhoit microwave process (2.54 GHz, 0.2 Torr to 0.3 Torr) which produced poor quality films at 20 to 30 Angstroms/sec. deposition rates.
Low pressure deposition, which is achieved with microwave plasmas without substantial increases in power allows for powderless depositions of a film with only small amounts of incorporated polymeric material. Still further, the higher frequency, and thus the more effective excitation results in the formation of reactive species not previously obtainable in sufficiently large concentrations with prior art processes. As a result, new amorphous semiconductor alloys can be produced having substantially different material properties than previously obtainable. All of the above results, by virtue of operation at microwave frequencies, and operation at lower pressures provide alloys having improved characteristics made at substantially increased rates. In addition, in order to operate in the most economic manner, it is further necessary to operate in the vicinity of the minimum of the Paschen curve.
As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374, new and improved amorphous and substantially amorphous semiconductor alloys and devices can be made which are stable and composed of chemical configurations which are determined by basic bonding considerations. One of these considerations is that the material is as tetrahedrally bonded as possible to permit minimal distortion of the material without long-range order. Fluorine, for example, when incorporated into these alloy materials performs the function of promoting tetrahedral bonding configurations. Amorphous semiconductor materials having such tetrahedral structures exhibit low densities of dangling bonds making the materials suitable for electronic and photovoltaic applications.
However, plasma deposited amorphous silicon-hydrogen alloys, as well as amorphous alloys of silicon and hydrogen with one or more of carbon, oxygen, and nitrogen suffer from various shortcomings. The hydrogen content of alloys deposited from hydride feed stocks is strongly dependent on substrate temperature, the content thereof decreasing at high substrate temperatures, and increasing at low substrate temperatures. For example, the presence of hydrogen in the alloy deleteriously effects the attainment of certain desirable properties of SiO.sub.2, Si.sub.3 N.sub.4, SiC, alloys or the like, such as corrosion resistance, hardness, etc. This is a direct consequence of hydrogen's role as a chemical bond terminator. As such, excess hydrogen disrupts the connectivity of the chemical bond network of the deposited film, reducing its average atomic coordination number. The hydrogen rich alloys have lower hardness, lower abrasion resistance, and lower corrosion resistance than stoichiometric materials.
Moreover, the use of pyrophoric hydride gases, as SiH.sub.4, introduces hazards into the work place which necessitate the introduction of extra manufacturing costs e.g., for worker safety aid protection systems, insurance and the like. These costs, while they may be already a part of the manufacturing costs of, for example, crystalline silicon semiconductor devices represent new costs for manufacturers who will use plasma deposited coatings in activities such as manufacturing and stamping data storage discs, manufacturing plastic headlight and tail light lenses, and manufacturing opthalmic lenses.
The presence of large amounts of hydrogen in the deposited alloy reduces the hardness of the deposited alloys, limiting their utility as protective coatings. Moreover, heating the substrate during deposition, e.g. to drive off hydrogen can damage temperatures sensitive substrates, as plastic substrates and substrates bearing electronic circuit elements and devices.
In the case of plastics, the inability to provide an adherent, abrasion resistant, substantially transparent coating impervious to moisture and oxygen on the plastic substrate without damaging or otherwise degrading the plastic substrate has limited the full potential of the transparent plastics and other soft substrates.
Plastics find many applications requiring the transmission, of optical energy. These applications include, for example, opthalmic and optical elements, windows and glazing units, and protective overlays, for example, protective overlays on optical data storage media and photovoltaically operatively activated devices.
Plastic overlays are used to protect optical data storage media. Information may be stored in the form of optically detectable differences in the properties of a medium, such as metal, an alloy, a phase change alloy, an organic or inorganic polymer, or an organic or inorganic dye. These media are particularly susceptible to environmental degradation, for example the effects of oxygen and moisture in the air. Heretofore these media have been protected by an organic polymeric overlay, as a poly (acrylate) overlay, e.g., poly (methyl methacrylate), or a poly (carbonate) overlay, as a bisphenol-A based heterochain polymeric overlay or an allyl carbonate heterochain polymeric overlay. However these optically transmissive polymers are water and air permeable and susceptible to surface degradation, as abrasion, scratching, and erosion. Even the slightest transport of air or water vapor across the barrier can force catastrophic failure of the information storage media. The surface degradation can cause skips, and incorrect data reading.
Plastic is also used as the refractive element in lenses, for example opthalmic lenses, photographic lenses, and telescope lenses. Especially preferred are polycarbonate and polyallyl carbonate polymers for opthalmic, sun glass, and safety goggle applications, and polymethyl methacrylate polymers for camera lenses, binocular lenses, telescope lenses, microscope objectives and the like. Plastic lenses have found good market acceptance and market penetration. However, the full potential of plastic lenses has been limited by their low resistance to abrasion, hazing, and scratching. Prior art abrasion resistant coatings, exemplified by polysilicate coatings and polysiloxane coatings, have not eliminated the problem of poor adhesion and poor abrasion resistance.
Plastic sheets with scratch and abrasion resistant coatings have found market acceptance in various automotive applications. These include headlight panels, parking light lenses, tail light lenses, sunroofs, side windows, and rear windows. However, the fuller utilization of plastic sheet material has been limited by the susceptibility of the plastic to scratching and abrasion, and the poor adhesion, and mismatch of thermal expansion coefficients between the plastic and the prior art protective coating.
Large area plastic sheets have also found utility in applications such as doors, windows, walls, air craft windows, air craft canopies, vandalism and break-in resistant panels, windows and doors, and esthetic barriers. However, the poor abrasion resistance of these large sheets have served to limit the full range of their market penetration capabilities.
Plastic materials have also been utilized to provide a shatter resistant layer for large sheets of glass. The glass-plastic structure is exemplified by bi-layer windshields. Bi-layer windshields have a single sheet of glass on the weather incident side of the windshield, and a polymeric film, for example a polyurethane film, adherent to the glass on the interior side. These bi-layer windshields have not found market acceptance because of the very poor resistance to scratching and abrasion of the internal, polyurethane coating thereof.
Other materials which require a hard coating are semiconductors, e.g., photosensitive semiconductors. Exposed layers of semiconductor alloy films, utilized in, for example, imagers, photovoltaic cells, electronic devices, and electrophotographic drums, are subject to abrasion, scratching, and hazing. Photovoltaic cells are subject to these insults during manufacturing and service, while imagers and electrophotographic drums are subject to the scratching, scraping, and abrading effects of rough sliding documents. In the case of electrophotographic drums, these effects are exacerbated by submicron, particulate toners.